Aim. We present a new illusory motion generated by a pattern drifted diagonally with respect to its two components. The components consist of (i) a set of horizontal bars spatially superimposed on (ii) a square-wave vertical grating. When the pattern is drifted diagonally, the two components can either appear to separate and slide past each other (sliding percept, illusory motion), or remain together as a single plaid-like pattern drifting diagonally (diagonal motion). We have investigated the chromatic conditions that determine the illusory motion. We show evidence that illusory motion occurs when the pattern is compatible with the chromatic conditions that determine perceptual transparency. Specifically, these conditions are defined by the invariance of the cone-excitation ratios (Ripamonti & Westland, 2003). That is, when a pair of surfaces (vertical grating) are viewed directly, then through a transparent filter (horizontal bars), the cone excitations for the pair of surfaces change, but the ratios of their excitations remain unchanged. Method. A series of horizontal bars (0.8deg) partially superimposed a square-wave vertical grating (0.8deg). The whole pattern moved diagonally at 10Hz and was viewed through a circular aperture (9.5deg). The cone-excitations of the grating and bars were manipulated in order to systematically vary the cone-excitation ratios. Observers responded whether the motion was sliding or diagonal. Results and Conclusions. When the two ratios approximated the invariance, observers perceived sliding motion. Moreover, the sliding percept was delayed due to the time taken for the colour and motion detecting mechanisms to interact.